LINQ to Dataset DBNULL问题/空引用异常 [英] LINQ to Dataset DBNULL problem / null reference exception
问题描述
我有以下LINQ查询,即使我测试它是否为NULL,当dtblDetail中的备注"列为null时,该查询始终会导致错误.
I have the following LINQ query which always results in an error when my "Remark" column in dtblDetail is null, even though I test if it is NULL.
var varActiveAndUsedElementsWithDetails =
from e in dtblElements
join d in dtblDetails on e.PK equals d.FK into set
from d in set.DefaultIfEmpty()
where (e.ElementActive == true)
select new
{
ElementPK = e.PK,
Remark = d.IsRemarkNull() ? null : d.Remark
};
错误消息是: 表'dtblDetails'中'注释'列的值为DBNull." 添加针对d.IsRemarkNull()的测试后,将引发空引用异常.
The error message was: "The value for column 'Remark' in table 'dtblDetails' is DBNull." After adding the test for d.IsRemarkNull() a null reference exception is thrown.
您能帮我吗?
我已经检查了以下网站,但除了必须测试DBNULL之外,没有发现其他有用的东西.但这不能解决我的问题.
I've already checked the following websites but didn't find anything useful other than that I have to test for DBNULL. But as said this doesn't solve my problem.
- http://social.msdn.microsoft.com/Forums/zh-CN/linqprojectgeneral/thread/3d124f45-62ec-4006-a5b1-ddbb578c4e4d
- http ://blogs.msdn.com/adonet/archive/2007/02/13/nulls-linq-to-datasets-part-3.aspx
- http://www.vbforums.com/showthread.php?t=506645
- http://social.msdn.microsoft.com/Forums/en-US/linqprojectgeneral/thread/3d124f45-62ec-4006-a5b1-ddbb578c4e4d
- http://blogs.msdn.com/adonet/archive/2007/02/13/nulls-linq-to-datasets-part-3.aspx
- http://www.vbforums.com/showthread.php?t=506645
推荐答案
问题是整个'd'项目为空. 因此,调用"d.IsRemarkNull()"会导致空引用异常. 以下代码解决了该问题:
The problem was that the whole 'd' item was empty. So calling 'd.IsRemarkNull()' resulted in the null reference exception. The following code fixed the problem:
var varActiveAndUsedElementsWithDetails =
from e in dtblElements
join d in dtblDetails on e.PK equals d.FK into set
from d in set.DefaultIfEmpty()
where (e.ElementActive == true)
select new
{
ElementPK = e.PK,
Remark = d == null? null : (d.IsRemarkNull() ? null : d.Remark)
};
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